This invention relates to hearing protective devices, such as ear plugs, and more particularly to an audiometric method and portable apparatus for testing in a field environment the noise attenuation and fitting of insert type of hearing protective devices fitted in the external ear canals.
The use of hearing protective devices, such as earmuffs and insert-type ear plugs or the like, are required by the Occupational Safety and Health Act of 1970 as a safety measure in the noise hazardous areas present in many industrial plants to prevent permanent hearing loss. In most instances, insert-type ear plugs are employed because of their light weight, compactness, and overall convenience. However, because ear canals vary considerably in size and shape, from ear to ear and from individual to individual it is difficult to obtain an effective fit. Standard ear plugs are ideally conceived and designed to fit a canal resembling the shape of a cylinder, not that of a cone or a squashed banana as the case may be. Yet in the field one finds all sorts of odd shapes and sizes of ear canals, some of which are difficult or impossible to fit with standard insert-type plugs. Even when the ear canals are custom fitted with ear plugs by professional audiology technicians, the real ear attenuation may fall below the predetermined standards, in the order of at least 20 dB across the sound spectrum.
Unfortunately, the conventional method of testing ear plugs for effectiveness and fitting described in American National Standard Institute (ANSI) standard Z24.22 requires the use of a research-type installation which is large and complicated, such as an anechoic chamber, is so costly that few activities can afford them solely for audiometric purposes. In addition it utilizes a common free field condition for both ears, being impossible to determine which one of the two ear plugs is defective. Furthermore, when the anechoic, or approved type test chamber is employed the head of the subject being tested must be held in a fixed position because of ear-to-speaker distance problems and other variables.
Persons working in noisy environments are psychologically adjusted to the noise so that they are mentally unaware of the danger of being exposed to the noise at the expense of their cochlea in the inner ear mechanism. Because of the lack of convenient and readily portable test apparatus, hearing losses caused by the false security of wearing an inadequately fitted ear plug in noise hazardous areas, the average hearing compensation claim in a U.S. Navy installation, for example amounts to $18,000.
Although earmuffs have been used for a different purpose, namely an absolute test of hearing, such as represented by U.S. Pat. No. 3,220,505, this type of earmuff is entirely unsuitable for the relative testing of ear plugs because the earmuffs are required, and constructed, to hug the ear tightly. Although the physical distortion of the ear canal caused by the earmuff in this patented device is not disadvantageous for reasons heretofore described it is critical for the purpose of the present invention.